Mesecraft as server has actually restarted many times, so there have been many maps that have been played through the years, so yes there has been many people to get down to the underworld and breach it’s surface. Back in the early versions of Mesecraft, the caves spawned way deeper than they are now.
Also, at the moment, Mithrel does not naturally spawn as ore in the world because the Mesecraft gods have other plans, however you can obtain it in loot chests in boats and underground.
On this Map I have only dug down to the Underworld, and am slowly working on mining down, mostly because I’m focused on building my estate. But in a previous map, a friend and I got down to the Nether.
(Also yes, someone has solved the puzzle chests and seals before, and as for the Ore Men? Those are so rare most players don’t even know they exist.)